The invention relates to a selective active low-pass filter and to a method for improving the selectivity of an active low-pass filter. It comes under the framework of the Multi-Radio-Front-End (MRFE) project for the design of multi-mode, multi-standard, fixed/mobile terminals integrating for example cellular telephone (GSM, UMTS etc.) systems, terrestrial digital television receiver (DVB-H/T) systems and systems for accessing local networks (WLAN a/b/g).
From a technical point of view, this convergence of various access modes into a single communicating object involves problems of coexistence due, in particular, to the proximity of the operating frequency bands of each of the modes.
The present invention is concerned, more particularly, with the coexistence of the DVB-H/T and GSM standards for which, as shown in FIG. 1, it is clear that the GSM signals transmitted within the band 890-915 MHz are definitely going to interfere with and degrade the DVB-T/H reception if no filtering device is incorporated into the system for isolating the 2 bands.
The very severe specification in terms of width leads necessarily to the use of an ultra-selective low-pass filter. Indeed, this filter must have a cut-off frequency higher than or equal to 862 MHz and reject the GSM band, from 890 to 915 MHz, by at least 20 dB. According to the results of a preliminary analysis, only the use of a filter of order 11 and possessing a response of the pseudo-elliptical type allows these objectives to be attained.
The network for this filter is shown in FIG. 1. The synthesis of this filter automatically leads to a symmetrical network structure. This comprises:                6 coupling inductors: 2*L1, 2*L2, 2*L3 connected in series between the input terminal E and the output terminal S;        and 5 series LC resonant elements: 2*Lr1/Cr1, 2*Lr2/Cr2, 1*Lr3/Cr3, inserted between the various coupling inductors and ground. These series LC elements resonate at frequencies that are very close to the cut-off frequency of the filter, and thus create transmission zeros which will allow the selectivity of the filter to be drastically enhanced.        
An application of the method of synthesis of this network recommended for this width of filter leads to the following values for the components:                Coupling inductors: L1=1.5 nH, L2=10 nH, L3=9.1 nH        Series L/C elements: Lr1=11 nH, Lr2=7.5 nH, Lr3=6.8 nH Cr1=2.2 pF, Cr2=3 pF, Cr3=3.3 pF        
An important remark for the following section relates to the resonance frequency of the L/C elements: It can indeed be noted that the L/C resonators which allow the transmission zeros to be obtained that are closest to the cut-off frequency are situated at the ends of the network, here in this case the 2 Lr1/Cr1 that resonate at the frequency Fr1=1023 MHz. The 2 other L/C resonators resonate at the frequencies Fr2=1061 MHz and Fr3=1062 MHz.
FIG. 2 shows the transmission response simulated by considering ideal L/C components. In this case, a cut-off frequency Fc around 860 MHz, shown by the point m1, and a rejection of the GSM band of greater than 40 dB, shown from the point m2, may effectively be noted.
The components usually employed are SMCs (for Surface-Mounted Components) such as offered, for example, by the manufacturer Murata from the series LQW15 for the inductors and from the series GRM15 for the capacitors.
In reality, if the parasitic elements are taken into account, the insertion losses are severely degraded in the neighbourhood of Fc. This degradation is mainly due to the parasitic series resistance (of typical value of around 1 ohm for wound SMC inductors) of the inductors which reduces the quality factor of the latter and consequently the performance of the filter in terms of insertion losses and selectivity. In FIG. 2, which shows the simulated performance of the filter taking into account the parasitic elements of the L/C components of the network, the degradation of the insertion losses at Fc is noted, said losses having risen to higher than 13 dB.